Term Paper on Mitochondria | Cell Organelle | Cell Biology

Here is a compilation of term papers on ‘Mitochondria’ for class 9, 10, 11 and 12. Find paragraphs, long and short term papers on ‘Mitochondria’ especially written for school and college students.

Term Paper on Mitochondria

Term Paper Contents:

1. Term Paper on the Discovery of Mitochondria
2. Term Paper on the Origin of Mitochondria
3. Term Paper on the Distribution of Mitochondria
4. Term Paper on the Plasticity of Mitochondria
5. Term Paper on the Morphology of Mitochondria
6. Term Paper on the Structure of Mitochondria
7. Term Paper on the Role of Mitochondria in Yolk Formation
8. Term Paper on the Mitochondrial Cristae
9. Term Paper on the Mitochondrial Particles
10. Term Paper on the Isolation of Mitochondria
11. Term Paper on the Isolation of Sub-Mitochondrial Components
12. Term Paper on the Mitochondrial Matrix
13. Term Paper on the Biochemistry of Mitochondria

Term Paper # 1. Discovery of Mitochondria:

Mitochondria are found in all eukaryotic cells capable of utilising oxygen. They are thus found in aerobically growing yeast, in protozoa, and in virtually every cell of higher plants and animals. These important organelles have been intensively studied for over a century, yet only in the past few years have we begun to understand how they actually function. And, of course, not all questions have been answered even now.

Perhaps the first description of mitochondria was by A. Kolliker, who in 1857 described them as the “sarcosomes” of muscle. Much later he was able to show that the mitochondria of muscle are individual entities and not connected directly to other parts of the cell. However, only after the application of appropriate staining procedures by Altmann in the latter years of the nineteenth century did it become possible to make detailed descriptions of mitochondrial distribution.

From such studies, Altmann concluded that mitochondria are autonomous organisms living within the cytoplasm of a host cell. Many doubted this theory and held to the modern notion that they are integral parts of the cell itself. Nevertheless, there is now the suspicion that mitochondria are, in fact, the highly revolved descendants of bacteria that once lived symbiotically in higher cells.

The function of these organelles was much debated during the early decades of this century. In 1912, B.F. Kingsbury proposed that they might be the sites of cellular respiration that is, the sites of oxygen utilisation.

He was quite right, of course, though direct proof had to await the development, primarily by George Palade and his associates at Rockefeller University, of improved methods of isolating cellular components. By 1950, however, it was recognised that mitochondria are not only the sites of cellular respiration, but also the major source of ATP production in aerobic animal cells.

Our understanding of the process whereby mitochondria are produced is still very incomplete.

Lehninger classified the various theories of possible routes of mitochondria) genesis into three main groups:

(1) Formation from other membranous structures in the cell,

(2) Growth and division of pre-existing mitochondria.

(3) De novo synthesis from submicroscopic precursors.

(1) Formation from Other Membranous Structures in the Cell:

The formation of mitochondria by “pinching off” or budding from pre-existing cell structures has been suggested for a range of cell membranes including those of the plasma membrane, endoplasmic reticulum, nuclear envelope, and Golgi complex. But the support for such evidence, in the absence of supporting biochemical data, cannot be wholly conclusive.

Part of the problem undoubtedly lies in our fragmentary knowledge of the structure and composition of and differences between cell memb­ranes in general. Indeed the similarities discussed in the literature between the mitochondrial membrane and the endoplasmic reticulum, could lend weight to the idea that probably mitochondria are formed when cytoplasm pushes into a cavity surrounded by an internal membrane, which then ‘pinches off’ and separates from the continuous system.

(2) Growth and Division of Pre-Existing Mitochondria:

Electron microscopic evidence mitochondrial division by fission, although plentiful is difficult to assess the danger of producing artifacts is very real because of the harsh chemical and physical agents brought to bear on the test material during processing. Interpretation is not made easier by the ability of mitochondria to undergo extreme changes in shape in vivo which may or may not be associated with mitochondrial fission.

There are numerous reports of mitochondria connected to each other by narrow bridges of membrane, especially in rapidly monopolising tissue, and it is thought that such figures may represent mitochondria in an early stage of fission.

By observing serial sections of rat liver Stempak (1967) was able to show that “dumb bell-shaped” mitochondria can be the sections of cup-shaped bodies. Such bodies have also been observed in rapidly growing tissues of fern and may represent the beginning stages of the division.

An early stage, mitochondrial division may involve the separation of mitochondrial contents into two or more compartments. The presence of mitochondria with internal “partitions” has been formed in several cell types although the possibility that they are manifestations of mitochondrial fusion cannot easily be ruled out.

Lafontaine and Allard have presented electron micrographs of rat liver mitochondria which exhibit what appear to be partitions dividing the inner membrane complex into two masses, the whole being surrounded by continuous outer membrane.

Tandler et al. has demonstrated the partitions of mitochondria in liver which was recovering from riboflavin deficiency.

(3) De Novo Synthesis:

The possibility of de-novo synthesis of mitochondria arose with experiments in the early part of the century, when mitochondria containing larvae were seen to develop from sea urchin egg cytoplasm which had apparently been freed of mitochondria by centrifugation Using the greater resolving power of the electron microscope, it was later shown, that mitochondria could not be dislodged by centrifugation of the egg.

Mitochondria had probably been present in the “centripetal end” of the egg cell after all, and these mitochondria could have served as precursors in subsequent mitochondrial production. A number of views were described, regarding the generation of mitochondria in the cytoplasm of different types of cells. But it is perhaps unwise to group the evidence to suit one or other of a limited number of clear cut methods by which mitochondria could replicate.

The actual situation is probably complex, and it may well be that different methods of replication take place in different tissues, and at different stages in development. One could imagine the early mitochondria being formed from membrane structures in the developing embryo concentration of mitochondria around the nuclear membrane have been noted in embryonic tissues from several phyla and formation of mitocho­ndria from this membrane could involve the transfer of nuclear genetic information essential for subsequent mitochondrial growth and multiplication by division.

The multiplication of mitochondria could then proceed by the incorporation of large pre-fabricated molecules and association of molecules, with division by fission, when the mitochondria reached a critical stage.

Prokaryotic Origin of Mitochondria:

The fact that mitochondria can grow, divide and are capable of mutations, support a long-held view that mitochondria originated with their host. Bacteria would have originated the mitochondria and blue green algae, the chloroplasts.

There are numerous homologies between mitochondria and bacteria. In bacteria the electron transport system is localised in the plasma membrane which can be compared with the inner membrane of mitochondria. Some bacteria even have membranous projections extending from the plasma membrane which are comparable to mitochondrial crests since both contain the respiratory chain.

The inner membrane and matrix, it has been postulated may represent the original symbiont which may become enclosed within a membrane of cellular origin (ER). Further mitochondrial DNA is circular; it replicates and divides like that of bacteria. The ribosomes are also found which are, however, smaller than those of the bacteria. In mitochondria and bacteria, the protein synthesis is inhibited by chloramp­henicol.

From these similarities, one can easily conceive of mitochondria as being evolved from an ancient prokaryote possessing all the attributes of an independent, probably aerobic organism.

However, with the adaptation over a long period it became an essential and dependent symbiont, lost same of its identity to the cell, and conversely, the host cell lost some of its functions, deriving it now from the endosymbiont or mitochondrion. As a result, both became obligatory symbionts to each other.

This symbiont hypothesis for the origin of mitochondria and plastids has achieved wide popularity, but all biologists do not necessarily accept it. Raff and Mahler concluded that “while the symbiotic theory may be esthetically pleasing, it is not compelling.”

They presented a lot of evidences and proposed that mitochondria arose by inward blobbing from plasma membrane, by the acquisition some-how of an outer membrane, and by the additional acquisition of a DNA genophore from the DNA of the proto-eukaryote in which the evolution of mitochondrion occurred. Borst proposed an episone theory and supposed that the DNA of mitochondrion left the ‘nuclear’ DNA by a sort of amplification to become mapped within a membrane containing the respiratory chain.

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Term Paper # 3. Distribution of Mitochondria:

Ordinarily mitochondria are evenly distributed in the cytoplasm. They may, however, be localised in certain regions. In proximal convoluted tubules of the kidney they are found in the basal region of the cell, opposite the renal capillaries. In skeletal muscles they lie between the myofibrils.

In insect flight muscle several large mitochondria are in contact with each fibril. In cardiac muscle the mitochondria are situated in clefts between the myofibrils, numerous lipid droplets are associated with the mitochondria. In many sperms the mitochondria fuse into one or two structures which lie in the middle piece of the tail, surrounding the aril filament. In columnar or prismatic cells they are oriented parallel to the long axis to the cell. In leucocytes they are radially arranged.

Arrangement of Mitochondria within the Cell:

Mitochondria often occur in close association with structures that either utilise the ATP they produce or provide the mitochondria with oxidisable substrates. Two striking examples of such arrangements are presented. In one, muscle cell mitochondria are seen lined up adjacent to the fibrils that utilise ATP during muscle contraction.

In the other, a mitochondrion is pictured surrounding a lipid droplet that contains fatty acids destined for mitochondrial oxidation. In thin- section electron micrographs mitochondria typically appear as ellipsoid or oval profiles measuring several micropeters in length and 0.5-1.0 μm across.

The widespread occurrence of the ellipsoid profile has led to the belief that lost mitochondria are sausage shaped, and that typical cells contain from several hundred to a few thousand such structures, however, this commonly held view has been challenged by fans-Peter.

Hoffman and Charlotte Avers, whose work on east cells has revealed that the three-dimensional shape of mitochondria within intact cells cannot be inferred from the examination of a few individual thin-section micrographs. After examining a consecutive series of micrographs obtained by thin-sectioning an entire yeast cell from one end to the other, these investigators were able to construct a three dimensional model of yeast mitochondria by assembling all the mitochondrial profiles observed in the individual sections.

The other surprising result was that the ellipsoid profiles observed in individual thin sections were all found to be derived from cross sections through a single large, extensively branched mitochondrion. When this experimental approach was applied to other eukaryotic cells, it was again found that many mitochondrial profiles seen in thin-section micrographs represent portions of larger, interconnected mitochondrial systems. Such results suggest that the number of mitochondria per cell is considerably smaller than once believed.

The conclusion that mitochondria form large interconnected systems is further supported by phase contrast microscopy of living cells, where the problem of sectioning is eliminated. Because the interconnected segments of these large, branched mitochondria appear to be in a state of flux, continually pinching off and re-fusing with one another, the concept of the “number” of mitochondria per cell may in fact be meaningless.

In isolated subcellular fractions, mitochondria appear small separate structures of relatively uniform size. It seem likely, however, that such individual “mitochondria” are artifacts caused by disruption of the branched, interconnected mitochondrial network during homogenisation. This type of membrane rearrangement is similar to the fragmentation process that causes endoplasmic reticulum membranes to be broken up into microsomal vesicles during subcellula fractionation.

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Term Paper # 4. Plasticity of Mitochondria:

Lewis and Lewis concluded that the mitochondria are extremely variable bodies, which are continually moving and changing shape in cytoplasm. There are no definite types of mitochondria, as any one type may change into another.

They appear to arise in cytoplasm and to be used up by cellular activity. The shape may change fifteen to twenty times in ten minutes; it can be changed by heat, hypetonic and hypotonic media or- by acids, fat solvents, potassium permanganate and osmotic changes. Frederic, Littre, Tobioka and Biesels studied the effect of large number of chemical and physical agents on mitochondrial behaviour. Some materials, such as detergents, show some effect in vivo as on mitochondria isolated from homogenates.

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Term Paper # 5. Morphology of Mitochondria:

Shape:

The shape is variable but is characteristic for a cell or tissue type, this too is dependable upon environment or physiological conditions. In general they are filamentous or granular. They may swell out at one end to become club-shaped or hollow out at one end to assume a shape of tents-racket. They may become vesicular by the appearance of central clear zone. Rod-shaped mitochondria are also observable.

Size:

The size of mitochondria also varies. In majority of the cells, width is relatively constant, about 0.5μ, but the length varies and, sometimes, reaches a maximum of 7μ. The size of cell also depends on the functional stage of the cell. Very thin mitochondria, about 0.2μ, or thick rods 2μ are also seen.

The size and shape of the fixed mitochondria are determined by the osmotic pressure and pH of the fixative. In acid, pH mitochondria are fragmented and become vesicular. Mitochondria, in the rat-liver, are usually 3.3μ in length; in mammalian exocrine pancreas, they are about 10μ in length and in oocytes of Amphibia, they are approximately of the length 20 to 40μ.

Number:

Mitochondria are found in the cytoplasm of all aerobically respiring cells with the exception of bacteria in which the respiratory enzymes are located in the plasma membrane. The mitochondria content of a cell is difficult to determine, but in general, it varies with the cell type and functional stage. It is estimated that in liver mitochondria constitute 30 to 35 per cent of the total protein content of the cell and it kidney, 20 per cent.

In lymphoid tissue the value is much lower. In mouse liver homogenates there are about 8.7 × 10¹⁰ mitochondria per gram of fresh tissue. A normal liver cell contains about 1000 to 1600 mitochondria, but this number diminishes during regeneration and also in cancerous tissue.

This last observation may be related to decreased oxidation that accompanies the increase to anaerobic glycolysis in cancer. Another interesting finding is that there is an increase in the number of mitochondria in the muscle after repeated administration of the thyroid hormone, thyroxin. An increased number of mitochondria have also been found in human hyperthyroidism. Thus cells with high metabolic activity have a high number of mitochondria, while those with low metabolic activity have a lower number.

Large sea urchin eggs have 13,000-14,000, while renal tubules have 300-400. In the sperm there are as few as 20-24 mitochondria while in some oocytes there are about 300,000. In the protozoon Chaos chaos there are about 500,000 mitochondria. Some algal cells contain only one mitochondrion.

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Term Paper # 6. Structure of Mitochondria:

A typical mitochondrion is sausage shaped with an average diameter of about 0.5μ. When it is properly fixed in osmium containing fluid and studied under electron microscope which reveals that there is hardly any difference between plant and animal mitochondria. In both the cases the mitochondrion is bounded by two membranes, the outer membrane and the inner membrane.

The space between the two membranes is called the outer chamber or inner-membrane space. It is filled with a watery fluid, and is 40-70 Å in width. The space bounded by the inner chamber is called the inner chamber or inner membrane space. The inner membrane space is filled with a matrix which contains dense granules (300-500 Å), ribosomes and mitochondria) DNA. The granules consist of insoluble inorganic salts and are believed to be the binding sites of divalent ions like Mg⁺⁺ and Ca⁺⁺.

In some cases they apparently contain polymers of sugars. The side of the inner membrane facing the matrix side is called the M-side, while the side facing the outer chamber is called the C-side. Two to six circular DNA molecules have been identified within mitochondria. These rings may either be in the open or in the twisted configuration. They may be present free in the matrix or may be attached to the membrane. The enzymes of the Krebs cycle are located in the matrix.

The inner membrane is thrown up into a series of folds, called cristae mitochondriales, which project into the inner chamber. The cavity of the cristae is called the inter-cristae space, and is continuous with the inter-membrane space.

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Term Paper # 7. Role of Mitochondria in Yolk Formation:

There have been a good number of investigations, whose account reveals that, mitochondria help in the formation of yolk in a developing ovum. The first study in this field was made by Loyez and latest probably by M.D.L. Srivastava, with the help of light microscope. The evidence adduced depends upon the topographical, and size relationship, and staining reactions of mitochondria and early protein yolk.

In the modern cytology with the investigation of electron microscope a new era has started and the studies of yolk formation do not remain away from electron microscope. With the help of electron microscope Farvard and Carasso come to the conclusion that mitochondria transformed into yolk granules in the egg of Planorbis coneus.

The main structural changes which they observed in the mitochondria are as follows:

(i) The cristae become disorganised in a few membranes, remaining concentric to the outer membrane before dropping completely.

(ii) In the matrix appeared a few minute granules which have scattered first, but become aggregated eventually in masses in regular pattern.

During Cell Division and Supermiogenesis:

Early cytologists, Benda, Dueberg and Meves were of opinion that mitochondria also divide equally during the cytoplasmic division and perhaps playing a part in inheritance.

Wilson commented that not the slightest proof has been produced of a fusion between the paternal and maternal chondrisomes. Frederic has briefly summarised various changes in mitochondria during cell division. The first phase shows a decrease in the total volume of mitochondrial material; gradually ceases its movements, pronounced thinning, fragmentation into small spheres, loss of optical density and finally assimilation into the cytoplasm.

In the second phase, when the cell divides into two, the modified mitochondria are separated passively, into the daughter cells. In the third phase, the modified mitochondria are reconstituted by addition of elements assimilated in the cytoplasm. Wilson found that in Opisthacanthus, during spermatogenesis, the number of mitochondria gradually reduces.

Pollister describes in Gerris that mitochondria arranged themselves into a well-defined ring, but without fusion. Modern microscopic studies provide firm conclusions regarding mitochondrial division during mitosis. Pyne noted in fowl adrenal cortex the mitochondria frequently appeared as pairs. This suggested that division, rather than fusion, was occurring.

In the transformation of spermatids into spermatozoa many mitochondrial changes are observed. Franzen observed in those sperms, which are shed directly in water, that mitochondria are present in form generally of four or five spheres below the sperm head, and in the case of sperms discharged in viscous medium these spheres transform into two elongated ribbion like filamentous mitochondria.

Sometimes these develop into ‘nebenkern spheres’ which may elongate and twist around the axial filament to form the mitochondrial sheath. Yasuzumi found an electron opaque body within ‘nebenkern’ of spermatids of Drosophilla; it is described as indistinguishable from a lipid droplet.

Term Paper # 8. Mitochondrial Cristae:

The space and arrangement of crests is variable and may be of the following types:

(i) Parallel to the long axis of mitochondria as in the neurons and the striated muscle cells.

(ii) Concentrically arranged as in the matrix of certain spermatids.

(iii) Interlaced to form villi as in Amoeba.

(iv) Cristae in the form of vesicles which form a network of interconnected chambers as in the cells of parathyroid gland and W.B.C. of man.

(v) Arranged in a tubular fashion but perpendicular to mitochondrial axis as in the cells of adrenal gland.

(vi) Haphazardly distributed as in the cells of kidney of insects and hepatic cells.

(vii) Cristae extremely small and irregular as in the interstitial cells of Opossum.

(viii) Rarely the mitochondrial wall is smooth with no cristae. The number and size of cristae in a mitochondrion directly affects its efficiency. The greater and larger are the cristae, the faster is the speed of oxidation reaction.

(ix) Perpendicular to the long axis of mitochondria.

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Term Paper # 9. Mitochondrial Particles:

The outer surface to the outer membrane and the inner surface of the inner membrane were supposed to be covered with thousands of small particles. Those on the outer membrane as being stalk less and were called the subunits of Parson. There may be as many as 10,000 to 100,000 particles per mitochondrion.

The stalked inner membrane particles were called the subunits of Fernandez-Moran, Elementary particles, F₁ particles or the oxiosomes or ETP or electron transport particles. These particles are about 85 Å in diameter and are regularly spaced at intervals of 10 nm on the inner membrane. There may be as many as 10⁴ to 10⁵ elementary particles per mitochondrion.

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Term Paper # 10. Isolation of Mitochondria:

Mitochondria can be isolated from the cell in the living form for their physiological studies. The cell first treated with deoxycholate for their break down. Then they are passed in sucrose solution.

The homogenate should be centrifuged for 10 minutes at the speed of 600 X g. From this homogenate upper substance is centrifuged at the speed of 8500 X g for 10 minutes. After this centrifugation, the upper microsomal fraction is discarded while the lower fraction consists of mitochondria and other particles like lysosomes.

This fraction is passed through sucrose gradient. The mitochondrial fraction then centrifuged at the speed of 10,000 X g upto 3 hrs. The upper part of this centrifuged material has mitochondria and lower part lysosomes.

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Term Paper # 11. Isolation of Sub-Mitochondrial Components:

The complex structural organisation of the mitochondrion raises many questions concerning the functional significance of the various components that make up this organelle. The development of methods for separating and isolating these mitochondrial components has played an important role in advancing our knowledge in this area.

The first successful technique for separating inner and outer mitochondrial membranes was developed by Donald Parsons and his colleagues in the 1960s. In this procedure mitochondria are placed in a hypotonic solution until the outer membrane ruptures, and the inner outer membranes are then separated from each other by isodensity centrifugation.

These two fractions can be readily distinguished from one another by electron microscopy; the isolated outer membranes look like empty sacs, while the isolated inner membranes form vesicles, called mitoplasts, which contain trapped matrix material within them.

The detergent digitonin has also been useful in isolating sub-mitochondrial fractions because it selectively disrupts the outer mitochondrial membrane and thus allows the outer and inner membranes to be separated from each other by centrifugation. In both of the above procedures the contents of the inter-membrane space are released into solution when the outer membrane is disrupted.

Hence any material appearing in the supernatant fraction after the initial centrifugation can be ascribed to the inter-membrane space. Once the mitoplast fraction has been isolated, it can be further separated into its membrane and matrix components by treating it with the detergent Lubrol, which disrupts the inner membrane, and re-centrifuging the resulting mixture.

Using this combination of techniques, the four major components of the mitochondrion can be separated from each other for biochemical analysis.

The highly permeable nature of the outer mitochondrial membrane causes the solute composition of the inter-membrane space to closely mirror that of the cell sap. The number of enzymes present in the inter-membrane space appears to be relatively small. Prominent among these is adenylate kinase, an enzyme that catalyses transfer of the terminal phosphate group of ATP to AMP, forming two molecules of ADP.

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Term Paper # 12. The Mitochondrial Matrix:

The matrix space contains all the enzymes and cofactors involved in the Krebs cycle with the single exception of succinate dehydrogenase, which is located in the inner membrane because it catalyses the direct transfer of electrons from the Krebs cycle intermediate, succinate, to the electron transfer chain.

Also present in the matrix are pyruvate dehydrogenase (which catalyses the conversion of pyruvate to acetyl-CoA), and the enzymes involved in fatty acid β-oxidation, which degrade fatty acids into acetyl-CoA units that enter the Krebs cycle. Analyses carried out on isolated matrix preparations have revealed that the protein concentration is too high for all the matrix proteins to be in true solution.

It has therefore been proposed that the enzymes of the Krebs cycle and fatty acid β-oxidation are anchored within a structural framework. This idea has received support from experiments carried out on mitochondria that have been artificially swollen by suspending them in a hypotonic medium. Under such conditions the matrix does not randomly disperse, but instead openings form in what appears to be an organised network. In electron micrographs the matrix generally exhibits a finely granular appearance, although large matrix granules ranging from 30 to several hundred manometers in diameter also occur.

In addition DNA, RNA, ribosomes, and other enzymes and factors involved in nucleic acid and protein synthesis are present in the matrix. Since those components are all involved in the process of mitochondrial growth and division.

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Term Paper # 13. Biochemistry of Mitochondria:

Lindberg and Ernster have given the data of chemical composition of mitochondria as follow:

a. Proteins 70 to 75%

b. Lipids 25-30%, and

c. RNA 5% of the dry weight.

But the recent biochemical analysis shows the following components:

(i) Proteins:

The proteins are the main constituents which are insoluble in water. The outer limiting membrane of mitochondria contains less than 10 percent of the total protein. There are about 14 different proteins having molecular weight from 12,000 to 22,000. The protein compositions of mitochondrial membranes are not fully known.

(ii) Lipid:

The lipid forms about 1/5th of the weight of the membranes. It is present almost entirely in the form of the molecules known as phospholipid. It has been reported by Meluick and Packer in 1971 that the outer membrane fraction is a 40% lipid contents as compared to 20% in the inner membrane.

(iii) Enzymes:

About 70 enzymes and 12 co-enzymes have been recognised in the mitochondria. Enzymes lie in a region as solid arrays, with perhaps as any as 5000 to 20,000 such assemblages in a single liver or heart mitochondria.

Mitochondrial DNA:

Recently the DNA is also reported from mitochondria. The mitochondrial DNA is double stranded like the nuclear 1NA. Each mitochondrion may contain one or more DNA molecules depending on its size, if, the mitochondrion is larger ten that may have more DNA molecules, having a circular shape.

Mitochondrial DNA differs from nuclear DNA in several aspects.

The GC content is higher in mitochondrial DNA, is consequently the buoyant density is also higher. Another difference is the higher denaturation temperature of mitochondrial DNA and facility with which it denatures.

The amount of genetic information carried by mitochondrial DNA not sufficient to provide specifications for all the proteins and enzymes present in this organoid. The most likely possibility is that mitochondrial DNA codes for some structural proteins.

Yeast mitochondria have been shown to contain DNA polymerase and more recently Kalf has succeeded in isolating the enzymes from rat liver mitochondria. Mitochondrial DNA polymerase appears to be involved in DNA replication rather than repair and possesses properties which are different from those of the nuclear enzymes.

These include a differing requirement for metal ion. Yeast mitoch­ondria, DNA polymerase appears to be, smaller than its nuclear counterpart, and is active at different stages of the cell cycle. Visual evidence showing what appears to be rat liver mitochondrial DNA in the process of replication has been presented by Kirschner, Wolsten holme and Gross.

Mitochondrial DNA does not appear to have histones associated with it as has nuclear DNA of higher organisms. In this respect mitochondrial DNA resembles with the bacterial DNA.

Mitochondrial RNA:

South and Mehlar suggested that the amount of mt-RNA is about 10 to 20 times that of mt-DNA. All sorts of RNA have been identified in mitochondria. The present evidences point out; conclusively the mitochondria contain complete set of t-RNA, aminoacyl RNA synthetases, as well as ribosomal RNA. All these components differ from their respective counterparts in the groundplasm.

The presence of m-RNA, transcribed from mitochondrial DNA is still uncertain. However, there are authorities who suggest its presence. The ribosomal RNAs’ are coded for by mitochondrial DNA and are thus apparently synthesized within the mitochondria by a mitochondrial DNA-dependent RNA polymerase system.

Mitochondrial Ribosome:

Mitochondria appear to contain ribosomes which are smaller in diameter than cytoplasmic ribosomes and Yeast mitochondria contain RNA species of 23S and 16S which would correspond to a 70S ribosome of bacterial type than rather the 80S ribosome of the cytoplasm.

Ribosome like particles with sedimentation values of 8IS and 55S have also been reported, and the extent of degradation suffered by the particles during isolation is not yet clear.

Polysomes like aggregation of ribosomes have been observed in sections of yeast mitochondria by Vignais, Huet and Andre in 1969. High molecular weight and RNA species associated with mitochondria which differ in sedimentation value from cytoplasmic ribosomal RNA have been reported in Yeast, Neurospora and He-La cells. Mitochondrial ribosomes require a higher concentration of Mg⁺⁺ ions to maintain their integrity than do cytoplasmic ribosomes.

Protein Synthesis:

In general, mitochondria can code any synthesize protein, but the DNA present therein is insufficient to code for all the proteins. It is suggested that mitochondria can synthesize the proteins of structural nature (cytochrome oxidase), but much of the proteins if not all, of the soluble proteins of the matrix as well as the proteins of the outer membrane and a number of proteins located in the cristae are under the control of nuclear DNA.

Of the proteins coded by nuclear DNA, it is generally agreed that the m-RNA derived from nucleus are translated in the cytoplasm, and the resulting proteins are then transported into the mitochondria. How then proteins enter the mitochondria?

Two methods have been proposed:

(1) The precursors enter the mitochondrion and inside are changed into the end products, thereby affecting a unidirectional flow a material into the mitochondria.

(2) There is the synthesis of lipoprotein vesicles which then merge and combine with the growing mitochondrion.